Allow rustc env vars to be set in files (#556)

Build scripts are executed in the exec configuration, not the target
configuration, which means it's currently impossible to use values which
were generated as a result of the `--stamp` flag when creating a
`rust_binary`. This means that embedding things like build timestamps,
git shas, and dynamic version numbers, is impossible.

This change allows you to depend, in the target configuration (because
no transition occurs on the attribute) on the output of a genrule which
consumes data from the stable and volatile workspace status commands.

docs/flatten.md

@@ -54,7 +54,7 @@ Produces a rust-project.json for the given targets. Configure rust-analyzer to l
 
 <pre>
 rust_benchmark(<a href="#rust_benchmark-name">name</a>, <a href="#rust_benchmark-aliases">aliases</a>, <a href="#rust_benchmark-compile_data">compile_data</a>, <a href="#rust_benchmark-crate_features">crate_features</a>, <a href="#rust_benchmark-crate_root">crate_root</a>, <a href="#rust_benchmark-data">data</a>, <a href="#rust_benchmark-deps">deps</a>, <a href="#rust_benchmark-edition">edition</a>,
-               <a href="#rust_benchmark-out_dir_tar">out_dir_tar</a>, <a href="#rust_benchmark-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_benchmark-rustc_env">rustc_env</a>, <a href="#rust_benchmark-rustc_flags">rustc_flags</a>, <a href="#rust_benchmark-srcs">srcs</a>, <a href="#rust_benchmark-version">version</a>)
+               <a href="#rust_benchmark-out_dir_tar">out_dir_tar</a>, <a href="#rust_benchmark-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_benchmark-rustc_env">rustc_env</a>, <a href="#rust_benchmark-rustc_env_files">rustc_env_files</a>, <a href="#rust_benchmark-rustc_flags">rustc_flags</a>, <a href="#rust_benchmark-srcs">srcs</a>, <a href="#rust_benchmark-version">version</a>)
 </pre>
 
 Builds a Rust benchmark test.
@@ -149,6 +149,7 @@ Run the benchmark test using: `bazel run //fibonacci:fibonacci_bench`.
 | <a id="rust_benchmark-out_dir_tar"></a>out_dir_tar |  __Deprecated__, do not use, see [#cargo_build_script] instead.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_benchmark-proc_macro_deps"></a>proc_macro_deps |  List of <code>rust_library</code> targets with kind <code>proc-macro</code> used to help build this library target.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_benchmark-rustc_env"></a>rustc_env |  Dictionary of additional <code>"key": "value"</code> environment variables to set for rustc.<br><br>rust_test()/rust_binary() rules can use $(rootpath //package:target) to pass in the location of a generated file or external tool. Cargo build scripts that wish to expand locations should use cargo_build_script()'s build_script_env argument instead, as build scripts are run in a different environment - see cargo_build_script()'s documentation for more.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {} |
+| <a id="rust_benchmark-rustc_env_files"></a>rustc_env_files |  Files containing additional environment variables to set for rustc.<br><br>These files should  contain a single variable per line, of format <code>NAME=value</code>, and newlines may be included in a value by ending a line with a trailing back-slash (<code>\</code>).<br><br>The order that these files will be processed is unspecified, so multiple definitions of a particular variable are discouraged.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_benchmark-rustc_flags"></a>rustc_flags |  List of compiler flags passed to <code>rustc</code>.   | List of strings | optional | [] |
 | <a id="rust_benchmark-srcs"></a>srcs |  List of Rust <code>.rs</code> source files used to build the library.<br><br>If <code>srcs</code> contains more than one file, then there must be a file either named <code>lib.rs</code>. Otherwise, <code>crate_root</code> must be set to the source file that is the root of the crate to be passed to rustc to build this crate.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_benchmark-version"></a>version |  A version to inject in the cargo environment variable.   | String | optional | "0.0.0" |
@@ -160,8 +161,8 @@ Run the benchmark test using: `bazel run //fibonacci:fibonacci_bench`.
 
 <pre>
 rust_binary(<a href="#rust_binary-name">name</a>, <a href="#rust_binary-aliases">aliases</a>, <a href="#rust_binary-compile_data">compile_data</a>, <a href="#rust_binary-crate_features">crate_features</a>, <a href="#rust_binary-crate_root">crate_root</a>, <a href="#rust_binary-crate_type">crate_type</a>, <a href="#rust_binary-data">data</a>, <a href="#rust_binary-deps">deps</a>,
-            <a href="#rust_binary-edition">edition</a>, <a href="#rust_binary-linker_script">linker_script</a>, <a href="#rust_binary-out_binary">out_binary</a>, <a href="#rust_binary-out_dir_tar">out_dir_tar</a>, <a href="#rust_binary-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_binary-rustc_env">rustc_env</a>, <a href="#rust_binary-rustc_flags">rustc_flags</a>,
-            <a href="#rust_binary-srcs">srcs</a>, <a href="#rust_binary-version">version</a>)
+            <a href="#rust_binary-edition">edition</a>, <a href="#rust_binary-linker_script">linker_script</a>, <a href="#rust_binary-out_binary">out_binary</a>, <a href="#rust_binary-out_dir_tar">out_dir_tar</a>, <a href="#rust_binary-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_binary-rustc_env">rustc_env</a>,
+            <a href="#rust_binary-rustc_env_files">rustc_env_files</a>, <a href="#rust_binary-rustc_flags">rustc_flags</a>, <a href="#rust_binary-srcs">srcs</a>, <a href="#rust_binary-version">version</a>)
 </pre>
 
 Builds a Rust binary crate.
@@ -267,6 +268,7 @@ Hello world
 | <a id="rust_binary-out_dir_tar"></a>out_dir_tar |  __Deprecated__, do not use, see [#cargo_build_script] instead.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_binary-proc_macro_deps"></a>proc_macro_deps |  List of <code>rust_library</code> targets with kind <code>proc-macro</code> used to help build this library target.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_binary-rustc_env"></a>rustc_env |  Dictionary of additional <code>"key": "value"</code> environment variables to set for rustc.<br><br>rust_test()/rust_binary() rules can use $(rootpath //package:target) to pass in the location of a generated file or external tool. Cargo build scripts that wish to expand locations should use cargo_build_script()'s build_script_env argument instead, as build scripts are run in a different environment - see cargo_build_script()'s documentation for more.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {} |
+| <a id="rust_binary-rustc_env_files"></a>rustc_env_files |  Files containing additional environment variables to set for rustc.<br><br>These files should  contain a single variable per line, of format <code>NAME=value</code>, and newlines may be included in a value by ending a line with a trailing back-slash (<code>\</code>).<br><br>The order that these files will be processed is unspecified, so multiple definitions of a particular variable are discouraged.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_binary-rustc_flags"></a>rustc_flags |  List of compiler flags passed to <code>rustc</code>.   | List of strings | optional | [] |
 | <a id="rust_binary-srcs"></a>srcs |  List of Rust <code>.rs</code> source files used to build the library.<br><br>If <code>srcs</code> contains more than one file, then there must be a file either named <code>lib.rs</code>. Otherwise, <code>crate_root</code> must be set to the source file that is the root of the crate to be passed to rustc to build this crate.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_binary-version"></a>version |  A version to inject in the cargo environment variable.   | String | optional | "0.0.0" |
@@ -534,7 +536,8 @@ rust_binary(
 
 <pre>
 rust_library(<a href="#rust_library-name">name</a>, <a href="#rust_library-aliases">aliases</a>, <a href="#rust_library-compile_data">compile_data</a>, <a href="#rust_library-crate_features">crate_features</a>, <a href="#rust_library-crate_root">crate_root</a>, <a href="#rust_library-crate_type">crate_type</a>, <a href="#rust_library-data">data</a>, <a href="#rust_library-deps">deps</a>,
-             <a href="#rust_library-edition">edition</a>, <a href="#rust_library-out_dir_tar">out_dir_tar</a>, <a href="#rust_library-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_library-rustc_env">rustc_env</a>, <a href="#rust_library-rustc_flags">rustc_flags</a>, <a href="#rust_library-srcs">srcs</a>, <a href="#rust_library-version">version</a>)
+             <a href="#rust_library-edition">edition</a>, <a href="#rust_library-out_dir_tar">out_dir_tar</a>, <a href="#rust_library-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_library-rustc_env">rustc_env</a>, <a href="#rust_library-rustc_env_files">rustc_env_files</a>, <a href="#rust_library-rustc_flags">rustc_flags</a>, <a href="#rust_library-srcs">srcs</a>,
+             <a href="#rust_library-version">version</a>)
 </pre>
 
 Builds a Rust library crate.
@@ -618,6 +621,7 @@ INFO: Elapsed time: 1.245s, Critical Path: 1.01s
 | <a id="rust_library-out_dir_tar"></a>out_dir_tar |  __Deprecated__, do not use, see [#cargo_build_script] instead.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_library-proc_macro_deps"></a>proc_macro_deps |  List of <code>rust_library</code> targets with kind <code>proc-macro</code> used to help build this library target.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_library-rustc_env"></a>rustc_env |  Dictionary of additional <code>"key": "value"</code> environment variables to set for rustc.<br><br>rust_test()/rust_binary() rules can use $(rootpath //package:target) to pass in the location of a generated file or external tool. Cargo build scripts that wish to expand locations should use cargo_build_script()'s build_script_env argument instead, as build scripts are run in a different environment - see cargo_build_script()'s documentation for more.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {} |
+| <a id="rust_library-rustc_env_files"></a>rustc_env_files |  Files containing additional environment variables to set for rustc.<br><br>These files should  contain a single variable per line, of format <code>NAME=value</code>, and newlines may be included in a value by ending a line with a trailing back-slash (<code>\</code>).<br><br>The order that these files will be processed is unspecified, so multiple definitions of a particular variable are discouraged.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_library-rustc_flags"></a>rustc_flags |  List of compiler flags passed to <code>rustc</code>.   | List of strings | optional | [] |
 | <a id="rust_library-srcs"></a>srcs |  List of Rust <code>.rs</code> source files used to build the library.<br><br>If <code>srcs</code> contains more than one file, then there must be a file either named <code>lib.rs</code>. Otherwise, <code>crate_root</code> must be set to the source file that is the root of the crate to be passed to rustc to build this crate.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_library-version"></a>version |  A version to inject in the cargo environment variable.   | String | optional | "0.0.0" |
@@ -727,7 +731,7 @@ See @rules_rust//proto:BUILD for examples of defining the toolchain.
 
 <pre>
 rust_test(<a href="#rust_test-name">name</a>, <a href="#rust_test-aliases">aliases</a>, <a href="#rust_test-compile_data">compile_data</a>, <a href="#rust_test-crate">crate</a>, <a href="#rust_test-crate_features">crate_features</a>, <a href="#rust_test-crate_root">crate_root</a>, <a href="#rust_test-data">data</a>, <a href="#rust_test-deps">deps</a>, <a href="#rust_test-edition">edition</a>, <a href="#rust_test-env">env</a>,
-          <a href="#rust_test-out_dir_tar">out_dir_tar</a>, <a href="#rust_test-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_test-rustc_env">rustc_env</a>, <a href="#rust_test-rustc_flags">rustc_flags</a>, <a href="#rust_test-srcs">srcs</a>, <a href="#rust_test-version">version</a>)
+          <a href="#rust_test-out_dir_tar">out_dir_tar</a>, <a href="#rust_test-proc_macro_deps">proc_macro_deps</a>, <a href="#rust_test-rustc_env">rustc_env</a>, <a href="#rust_test-rustc_env_files">rustc_env_files</a>, <a href="#rust_test-rustc_flags">rustc_flags</a>, <a href="#rust_test-srcs">srcs</a>, <a href="#rust_test-version">version</a>)
 </pre>
 
 Builds a Rust test crate.
@@ -877,6 +881,7 @@ Run the test with `bazel build //hello_lib:hello_lib_test`.
 | <a id="rust_test-out_dir_tar"></a>out_dir_tar |  __Deprecated__, do not use, see [#cargo_build_script] instead.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | optional | None |
 | <a id="rust_test-proc_macro_deps"></a>proc_macro_deps |  List of <code>rust_library</code> targets with kind <code>proc-macro</code> used to help build this library target.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_test-rustc_env"></a>rustc_env |  Dictionary of additional <code>"key": "value"</code> environment variables to set for rustc.<br><br>rust_test()/rust_binary() rules can use $(rootpath //package:target) to pass in the location of a generated file or external tool. Cargo build scripts that wish to expand locations should use cargo_build_script()'s build_script_env argument instead, as build scripts are run in a different environment - see cargo_build_script()'s documentation for more.   | <a href="https://bazel.build/docs/skylark/lib/dict.html">Dictionary: String -> String</a> | optional | {} |
+| <a id="rust_test-rustc_env_files"></a>rustc_env_files |  Files containing additional environment variables to set for rustc.<br><br>These files should  contain a single variable per line, of format <code>NAME=value</code>, and newlines may be included in a value by ending a line with a trailing back-slash (<code>\</code>).<br><br>The order that these files will be processed is unspecified, so multiple definitions of a particular variable are discouraged.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_test-rustc_flags"></a>rustc_flags |  List of compiler flags passed to <code>rustc</code>.   | List of strings | optional | [] |
 | <a id="rust_test-srcs"></a>srcs |  List of Rust <code>.rs</code> source files used to build the library.<br><br>If <code>srcs</code> contains more than one file, then there must be a file either named <code>lib.rs</code>. Otherwise, <code>crate_root</code> must be set to the source file that is the root of the crate to be passed to rustc to build this crate.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
 | <a id="rust_test-version"></a>version |  A version to inject in the cargo environment variable.   | String | optional | "0.0.0" |